Piezoelectric MEMS actuator for compensating unwanted movements and manufacturing process thereof

ABSTRACT

A MEMS actuator includes a monolithic body of semiconductor material, with a supporting portion of semiconductor material, orientable with respect to a first and second rotation axes, transverse to each other. A first frame of semiconductor material is coupled to the supporting portion through first deformable elements configured to control a rotation of the supporting portion about the first rotation axis. A second frame of semiconductor material is coupled to the first frame by second deformable elements, which are coupled between the first and the second frames and configured to control a rotation of the supporting portion about the second rotation axis. The first and second deformable elements carry respective piezoelectric actuation elements.

PRIORITY CLAIM

This application claims the priority benefit of Italian Application forPatent No. 102019000007219, filed on May 24, 2019, the content of whichis hereby incorporated by reference in its entirety to the maximumextent allowable by law.

TECHNICAL FIELD

This disclosure relates to a Micro-Electro-Mechanical System (MEMS)piezoelectric actuator for compensating unwanted movements and to amanufacturing process thereof. In particular, hereinafter reference ismade to a piezoelectric MEMS actuator configured to carry out opticalimage stabilization (OIS) in optical devices such as, for example,digital still cameras (DSCs), in particular for autofocus applications,without this disclosure being limited thereto.

BACKGROUND

As is known, actuators are devices that convert a physical quantity ofone type into another physical quantity of a different type; inparticular, the quantity deriving from conversion usually leads to someform of movement or mechanical action.

Recently, actuators of micrometric and nanometric dimensions have beenproposed, also referred to as micro-actuators or nano-actuators, whichcan be manufactured using a semiconductor technology (for example, MEMStechnology) and therefore at contained costs. Such micro-actuators andnano-actuators may be used in a wide range of devices, in particular inmobile and portable devices.

Examples of micro-actuators are valves, switches, pumps, linear androtary micromotors, linear positioning devices, speakers and opticaldevices (for example, optical autofocus devices).

Known micro-actuators may work according to four physical principles:

Electrostatic: they exploit the attraction between conductors that areoppositely charged;

Thermal: they exploit the displacement caused by thermal expansion orcontraction;

Piezoelectric: they exploit the displacement caused by stresses andstrains induced by electrical fields; and

Magnetic: they exploit the displacement caused by the interactionbetween different elements having magnetic characteristics, such aspermanent magnets, external magnetic fields, magnetizable materials, andelectric current conductors.

Each technology has advantages and limits in terms of power consumption,movement rapidity, force exerted, movement amplitude, movement profile,simplicity of manufacture, amplitude of the applied electrical signals,robustness, and sensitivity, which render use thereof advantageous incertain applications, but not in others and therefore determine theirfield of use.

Hereinafter a MEMS actuator operating according to the piezoelectricprinciple and in particular exploiting MEMS thin-film piezo (TFP)technology is considered.

TFP MEMS technology currently uses a unimorphic mode of actuation,wherein a structure (for example, a membrane, a beam or a cantilever),usually formed of at least two superimposed layers, undergoes bending asa result of variations in the applied stress. In this case, a controlledalteration of the strain is obtained in one of the layers, referred toas active layer, which causes a passive strain in the other layer orlayers, referred to also as inactive or passive layers, with consequentbending of the structure.

The above technique is advantageously used for bending the membrane, thebeam, or the cantilever in applications where it is desired to obtain avertical movement, i.e., a movement in a direction perpendicular to theplane in which the structure lies, for example, in an ink printhead,autofocus systems, micro-pumps, microswitches and speakers.

For instance, FIGS. 1A and 1B show a cantilever beam 1 constrained at afirst end 2 and free to bend at a second end 3. The beam 1 is hereformed by a stack of layers including: a supporting layer 5, forexample, of semiconductor material with a first conductivity type, forexample, P; an active layer 6, for example, of piezoelectric material(PZT); and a top layer 7, for example, of semiconductor material with asecond conductivity type, for example, N.

In presence of a reverse biasing, as illustrated in FIG. 1B, the appliedelectrical field causes strains in the beam 1, which causes a movementof the second end 3 downwards.

An embodiment of a piezoelectric MEMS actuator applied to a genericoptical device is illustrated in FIGS. 2A and 2B. Here, the opticaldevice, designated by 10, comprises a deformable part or membrane 15,for example of glass, such as BPSG (BoroPhosphoSilicate Glass), resting,through a lens element 11 (for example of polymeric material), on asupport 12, which is also, for example, of glass; the membrane 15further carries two piezoelectric regions 13, arranged at a mutualdistance apart.

In absence of biasing, in FIG. 2A, the membrane 15 and the lens element11 have planar surfaces and do not modify the path of a light beam 16that passes them. When the piezoelectric regions 13 are biased, in FIG.2B, they cause a deformation of the membrane 15. The deformation of thecentral area of the membrane 15 is transmitted to the lens element 11,whose top surface bends, modifying the focus of the lens element 11 andtherefore the path of the light beam 16. It is thus possible to modifythe characteristics of optical transmission of the optical device 10.

It is furthermore known that known optical devices, such as digitalstill cameras, may be subject, in use, to unwanted movements inducedfrom outside, such as vibrational movements induced by quivering of theuser's hand that is using the digital still camera.

In particular, in use, one or more lenses of the optical device receivea light beam and focus it towards an image sensor, housed in the opticaldevice; next, the image sensor receives and processes the focused lightbeam to generate an image.

However, when the optical device is subjected to unwanted movements, theoptical path of the light beam through the lenses towards the imagesensor is deflected; consequently, the image sensor receives the lightbeam in a shifted position with respect to the case with no movementsinduced from outside. Consequently, the image sensor may generate a lowquality image, for example, an out-of-focus image.

To address this issue, in the last few years optical devices integratingactuators and corresponding sensing systems configured to quantify andcompensate the unwanted movements have been developed.

For instance, U.S. Pat. No. 9,625,736, incorporated by reference,describes an actuator of the type schematically represented in FIGS.3A-3B. In particular, FIG. 3A shows an example of a portion of anoptical device 30 (e.g., a digital still camera) including an actuator40 for compensating unwanted movements induced from outside andgenerating displacements along an X axis and an Y axis of a Cartesianreference system XYZ. In the example illustrated, the actuator 40 is avoice coil motor (VCM), i.e. an electromagnetic actuator.

The optical device 30 comprises a supporting structure 32, comprising acasing 52 (not shown in FIG. 3B for clarity reasons), and a substrate 42defining a first and a second surface 32A, 32B. The supporting structure32 houses a first cavity 34, in communication with the externalenvironment through an opening 36 formed in the casing 52 at the firstsurface 32A. In particular, the cavity 34 houses the actuator 40.

The substrate 42, of semiconductor material (e.g. polysilicon), has arecess 44 facing the outside of the supporting structure 32 and housinga first printed circuit board (PCB) 46.

The first printed circuit board 46 carries a movement sensor 48 and anintegrated driving circuit 49, electrically coupled together throughconductive paths (not illustrated).

A first, a second, a third, and a fourth permanent magnetic element51A-51D are arranged within the supporting structure 32 and have, forexample, a parallelepipedal shape with a reduced thickness in top view(FIG. 3B). In particular, the first and the second permanent magneticelements 51A, 51B have a magnetization opposite to each other, extend ontwo opposite sides of the supporting structure 32 and have longer sides(in the top view of FIG. 3B) parallel to the X axis of the Cartesianreference system XYZ; likewise, the third and the fourth permanentmagnetic elements 51C, 51D have a magnetization opposite to each other,extend on further two opposite sides of the supporting structure 32, andhave longer sides (in the top view of FIG. 3B) parallel to the Y axis ofthe Cartesian reference system XYZ.

The permanent magnetic elements 51A-51D are arranged along the sidewalls of the casing 52, inside it, and surround the actuator 40 at adistance.

The casing 52 extends alongside and over the permanent magnetic elements51A-51D, as well as at least in part extends laterally with respect tothe substrate 42 (FIG. 3A).

The optical device 30 further comprises an image acquisition module 38,including a first and a second optical module 60, 61, coaxial to eachother and having an optical axis S parallel to the Z axis.

In detail, the first module 60 comprises a first lens 70, configured toreceive a light beam 72 from the external environment. The second module61 comprises second lenses 71 (three schematically shown in FIG. 3 ),optically coupled to the first lens 70.

The image acquisition module 38 is accommodated in a barrel 80, in acavity 81 thereof.

Moreover, the optical device 30 comprises a second printed circuit board82, coupled to the barrel 80 at a top surface 82A thereof to delimit thecavity 81 of the barrel 80 at the bottom. An image sensor 84 extends onthe top surface 82A of the second printed circuit board 82; for example,the image sensor 84 is formed by an array of diodes and is electricallycoupled to the second printed circuit board 82. Moreover, the imagesensor 84 is operatively coupled to the image acquisition module 38; inparticular, the first and the second optical modules 60, 61 areconfigured to focus the light beam 72 on the image sensor 84.

The actuator 40 of the optical device 30 comprises a magnetic body 90(e.g. of ferromagnetic material), surrounding the barrel 80, and a coil92, extending around the magnetic body 90 and electrically coupled tothe integrated driving circuit 49 by conductive paths (not shown).

In use, when the optical device 30 is subject to unwanted movementsinduced from outside, the movement sensor 48 detects these movements andgenerates an electrical signal, which is transmitted to the integrateddriving circuit 49; the integrated driving circuit 49 processes theelectrical signal and determines, for example, the magnitude anddirection of the force generated by the movements on the optical device30.

On the basis of the processed information, the integrated drivingcircuit 49 generates a current that is fed to the coil 92 to move theimage detection structure 38 along the X and Y axes.

In detail, as a result of passage of the current in the coil 92, aLorentz force acts between the actuator 40 and the permanent magneticelements 51A-51D and causes movement of the image acquisition module 38,together with the barrel 80 and the second printed circuit board 82,toward the first or the second permanent magnetic element 51A, 51B(movement along X) and/or towards the third or the fourth permanentmagnetic element 51C, 51D (movement along Y).

Consequently, the light beam 72 is deflected by an angle correlated tothe magnitude of the Lorentz force, compensating the unwanted movements.

The actuator 40 of the optical device 30 enables a correction of theoptical path of the light beam 72 by an angle that, for medium-leveldigital still cameras is, for example, ±0.75° and, for professionaldigital still cameras, is, for example, ±1.50°.

However, optical devices of the type illustrated in FIG. 3 have somedisadvantages.

In particular, the actuator 40 moves the image acquisition module 38 ata limited speed, since the electromagnetic actuation is slow.

Moreover, the current used by the coil 92 of the actuator 40 to generatea Lorentz force sufficient to compensate the unwanted movements inducedfrom outside is high (e.g. comprised between 50 mA and 80 mA).

There is a need in the art to provide a MEMS actuator and amanufacturing process therefore that overcome the drawbacks of the priorart.

SUMMARY

According to this disclosure, a MEMS actuator and a manufacturingprocess thereof are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding, embodiments thereof are now described,purely by way of non-limiting example, with reference to the attacheddrawings, wherein:

FIGS. 1A and 1B show simplified side views of a known piezoelectric MEMSactuator in a resting condition and in a deformation condition,respectively;

FIGS. 2A and 2B show simplified side views of another knownpiezoelectric MEMS actuator, used in an optical device in a restingposition and in a deformation condition, respectively;

FIG. 3A is a schematic cross-section of a known optical device;

FIG. 3B is a schematic top view with removed parts of the optical deviceof FIG. 3A;

FIG. 4 shows a top view of the present MEMS actuator according to afirst embodiment;

FIG. 5 is a perspective bottom view of the MEMS actuator of FIG. 4 ;

FIG. 6 is a longitudinal section of a part of the MEMS actuator of FIG.4 , taken along section line VI-VI of FIG. 4 ;

FIGS. 7 and 8 are perspective views of the MEMS actuator of FIG. 4 , indifferent operating positions;

FIG. 9 is a top view of the present MEMS actuator according to anotherembodiment;

FIG. 10 is a top view of the present MEMS actuator according to afurther embodiment; and

FIGS. 11-15 are cross-sections through a portion of the MEMS actuator ofFIGS. 3, 9, and 10 , in subsequent manufacturing steps.

DETAILED DESCRIPTION

FIGS. 4-6 show schematically a MEMS actuator 100 of a piezoelectrictype. In particular, the MEMS actuator 100 is configured to integrateoptical devices, for example for autofocus, and allows compensation ofunwanted movements.

The MEMS actuator 100 is formed by a monolithic body 101 ofsemiconductor material (e.g., polysilicon) having a generallyparallelepipedal shape with a first and a second larger surface 100A,100B and a reduced thickness (in a direction parallel to a Cartesianaxis Z of a Cartesian reference system XYZ). In the embodiment of FIGS.4-6 , the MEMS actuator 100 has (in top view) a square shape, with aside of, for example, 7 mm×7 mm and a depth (in the Z direction) of, forexample, 710 μm.

The body 101 of the MEMS actuator 100 comprises a supporting portion 102having, in top view (FIG. 4 ), a quadrangular shape (for example,square); and a first frame 104 surrounding the supporting portion 102,having a polygonal shape in top view (for example, of an elongatedhexagon), and coupled to the supporting portion 102 by first deformableelements 115; and a second frame 108 surrounding the first frame 104,having, in top view, a quadrangular shape (for example, square), andcoupled to the first frame 104 by second deformable elements 116. Thesecond frame 108 is here rotated through 45° with respect to the firstframe 104.

In particular, in the embodiment shown in FIGS. 4-6 , the two diagonalsof the second frame 108 form a first and a second symmetry axis A, B ofthe supporting portion 102, transverse (in particular, perpendicular)with respect to each other and further forming symmetry axes for thefirst frame 104, which is elongated in the direction of the firstsymmetry axis A. In particular, the first frame 104 has two longer sides104A, parallel to each other and to the first symmetry axis A, and fourshorter sides 104B (parallel two by two), transverse with respect to thesymmetry axes A and B (here set at 45°). The longer sides 104A of thefirst frame 104 are therefore transverse (at 45°) with respect to thesides of the second frame 108, and the shorter sides 104B of the firstframe 104 are parallel (two by two) to the sides of the second frame108.

In particular, the symmetry axes A, B intersect each other at a center Oand lie in an XY plane of the Cartesian reference system XYZ, similar tothe larger surfaces 100A, 100B of the MEMS actuator 100, due to thenegligible depth of the MEMS actuator 100 (along the axis Z).

The supporting portion 102 has an opening 120 having, for example, acircular shape, with center O at the center of the second frame 108 andof the MEMS actuator 100.

The MEMS actuator 100 carries a lens 125 of transparent material (e.g.,glass, such as BPSG, silicon oxide, or PSG) bonded, for example glued,to the supporting portion 102 on the second surface 100B of the actuatorand here having a parallelepipedal shape. In greater detail, the opening120 is configured to enable, in use, the passage of a light beam throughthe lens 125.

The first deformable elements 115 comprise a first and a second springelement 106, 107; moreover, the second deformable elements 116 comprisethird and fourth spring elements 110, 111.

The first and the second spring elements 106, 107 are symmetrical toeach other with respect to the second symmetry axis B, are fixed to thesupporting portion 102 by respective first ends 106A, 107A, and fixed tothe first frame 104 by respective second ends 106B, 107B. In theembodiment illustrated in FIGS. 4 and 5 , the first and the secondspring elements 106, 107 have a serpentine shape.

In particular, the first and the second spring elements 106, 107comprise respective first and second deformable arms 130, 132 andrespective first and second connection arms 131, 133, which extendparallel to each other and to the second symmetry axis B, as well asperpendicular to the first symmetry axis A. The first and the secondconnection arms 131, 133 are interposed between two respective first andsecond deformable arms 130, 132 successive to each other along theserpentine shape (in a direction parallel to the first symmetry axis A).In particular, each connection arm 131, 133 connects subsequent ends ofthe deformable arms 130, 132 arranged on opposite sides of the firstsymmetry axis A.

Likewise, the third and the fourth spring elements 110, 111 aresymmetrical to each other with respect to the first symmetry axis A andare fixed to the first frame 104 at respective first ends 110A, 111A andto the second frame 108 at respective second ends 110B, 111B. In theembodiment illustrated in FIGS. 4 and 5 , the third and the fourthspring elements 110, 111 also have a serpentine shape.

Similarly to the first and the second spring elements 106, 107, thethird and the fourth spring elements 110, 111 comprise each respectivethird and fourth deformable arms 140, 142 and respective third andfourth connection arms 141, 143, extending parallel to each other and tothe first symmetry axis A, as well as perpendicular to the secondsymmetry axis B. The third and the fourth connection arms 141, 143 areinterposed between respective successive third and fourth deformablearms 140, 142 (in a direction parallel to the second symmetry axis B) toform the serpentine structure.

The first ends 106A, 107A of the first and the second spring elements106, 107 are fixed to the supporting portion 102 in a symmetricalposition with respect to the second symmetry axis B, spaced at adistance from, and on the same side of, the first symmetry axis A, forexample, in proximity to the third spring element 110. Moreover, thesecond ends 106B, 107BA of the first and the second spring elements 106,107 are fixed to the first frame 104 at two respective shorter sides104B, in a symmetrical position with respect to the second symmetry axisB, and are spaced at a distance from, and on the same side of, the firstsymmetry axis A, here in proximity to the fourth spring element 111.

Likewise, the first ends 110AB, 111A of the third and the fourth springelements 110, 111 are fixed to the first frame 104 at two respectivelonger sides 104A, in a position symmetrical with respect to the firstsymmetry axis A, and are spaced at a distance from, and on the same sideof, the second symmetry axis B, for example, adjacent to the firstspring element 106. Moreover, the second ends 110B, 111B of the thirdand the fourth spring elements 110, 111 are fixed to the second frame108 in a position symmetrical with respect to the first symmetry axis A,and are spaced at a distance from, and on the same side of, the secondsymmetry axis B, here in a position adjacent to the second springelement 107.

Due to the arrangement of the deformable arms 130, 132, 140, 142 and ofthe connection arms 131, 133, 141, 143 transverse to the sides of thesecond frame 108, in proximity to the corners of the latter, they havevariable lengths, as may be seen in FIGS. 4 and 5 .

As may be seen in particular in FIG. 5 , the deformable arms 130, 132,140, 142 have a smaller thickness than the rest of the body 101, exceptat their own ends, so as to have a high flexibility, as described indetail hereinafter with reference to FIG. 6 . For instance, they mayhave a thickness comprised between 4 μm and 100 μm, in particular here80 μm. Each of the deformable arms 130, 132, 140, 142 carries arespective strip 150 of piezoelectric material, for example of a ceramicwith a base of lead-titanate-zirconate (PZT) or of aluminum nitride(AlN).

FIG. 6 shows the structure of the first deformable arm 130; thisstructure is identical also for the deformable arms 132, 140, 142.Furthermore, FIG. 6 also shows a portion of the first frame 104 (inparticular, of one of the longer sides 104A).

In detail, the deformable arm 130 comprises a first and a secondsubstrate portion 702A, 702B, laterally delimiting a cavity 810. Thelonger side 104A comprises a third substrate portion 702C, laterallydelimiting, together with the second substrate portion 702B, a trench755.

A first insulating layer 704, for example, of silicon oxide, extends onthe substrate portions 702A-702C.

A membrane layer 706, of semiconductor material (e.g., polysilicon),extends on the first insulating layer 704; in particular, it ispartially suspended over the cavity 810 to form here a membrane 812(portion of reduced thickness, also visible in FIG. 5 ).

A second insulating layer 180, for example of silicon oxide, extends atleast in part over the membrane layer 706.

The strip 150 extends on the second insulating layer 180; in particular,the first strip 150 comprises a stack formed by a first electrode 171, apiezoelectric region 172 and a second electrode 173. The strip 150 formsa capacitor. In use, the first electrode 171 is connected to a referencepotential (for example, ground) and the second electrode 173 isconnected to a voltage source 200 through first conductive paths 210(schematically illustrated in FIG. 4 ).

A first passivation layer 730, for example of aluminum oxide, extends onthe first insulating layer 180 and on the first and the secondelectrodes 171, 173, as well as alongside the piezoelectric region 172;moreover, a second passivation layer 732, for example, of USG (UndopedSilicon Glass), extends over the first passivation layer 730. Inparticular, a first and a second contact opening 740, 741 extend throughthe first and the second passivation layers 730, 732 and expose portionsof the first and the second electrodes 171, 173, respectively, of thestrip 150.

A first and a second metallization layer 734A, 734B, of conductivematerial, extend on the second passivation layer 732 and in the contactopenings 740, 741 to electrically contact the first and the secondelectrodes 171, 173.

A third passivation layer 736, for example, of nitride, extends on thesecond passivation layer 732 and on the first and the secondmetallization layers 734A, 734B. A third contact opening 750 extendsthrough the third passivation layer 736 and exposes a portion of thefirst metallization layer 734A.

A contact layer 752, of conductive material (for example, gold, Au),extends on the third passivation layer 736 and fills the third contactopening 750 to electrically contact the first metallization layer 734A.

With reference once again to FIG. 4 , the strips 150 of the first andthe second deformable arms 130, 132 are biased by the first voltagesource 200 through the first conductive paths 210, and the strips 150 ofthe third and the fourth deformable arms 140, 142 are electricallyconnected to a second voltage generator 202 through second conductivepaths 212 (schematically illustrated in FIG. 4 ).

Application of a static actuation voltage (for example of 40 V) to thestrips 150 of the third and the fourth deformable arms 140, 142 causesan upward deflection of the latter out of the XY plane; moreover, due tothe absence of a bias of the third and the fourth connection arms 141,143, these do not undergo deformation but rigidly rotate with the thirdand the fourth deformable arms 140, 142, respectively. Consequently, byvirtue of also of the serpentine shape of the third and the fourthspring elements 110, 111, the first frame 104, the first spring element106, the second spring element 107 and the supporting portion 102 rotateapproximately about the second symmetry axis B, as illustrated in FIG. 7(where the MEMS actuator 100 is shown rotated by 90° counterclockwisewith respect to the top view of FIG. 5 ).

Likewise, by applying a static actuation voltage (for example of 40 V)to the strips 150 of the first and the second deformable arms 130, 132,it is possible to obtain a rigid rotation of the first and the secondconnection arms 131, 133 with the first and the second deformable arms130, 132, respectively, as well as rotation of the supporting portion102 approximately about the first symmetry axis A, as illustrated inFIG. 8 (where the MEMS actuator 100 is shown rotated through 90°counterclockwise with respect to the top view of FIG. 5 ).

By simultaneously biasing all the strips 150 and modulating theactuation voltage applied to them, it is possible to rotate thesupporting portion 102 about both the rotation axes A, B by a selectableangle (up to a maximum value of, for example, 1.2°).

FIG. 9 shows another embodiment of the present MEMS actuator. In detail,FIG. 9 shows a MEMS actuator 300 having a general structure similar tothe MEMS actuator 100 illustrated in FIGS. 4-6 , so that parts similarto the ones illustrated and described with reference to FIGS. 4-6 aredesignated in FIG. 9 by reference numbers increased by 200 and will notbe described any further.

In the embodiment of FIG. 9 , the first and the second spring elements306, 307 have first and second ends 306A, 307A and 306B, 307B and arearranged on opposite sides of both the first and the second symmetryaxis A, B respectively; in general, the second spring element 307 may beobtained by rotating the first spring element 306 through 180° withrespect to the center O. Likewise, the third and the fourth springelements 310, 311 have first and second ends 310A, 311A and 310B, 310Band arranged, respectively, on opposite sides both of the first and ofthe second symmetry axis A, B; in general, the fourth spring element 311may be obtained by rotating the third spring element 310 through 180°with respect to the center O.

In the present embodiment, each of the strips 350 is electricallyconnected to a respective voltage source 400-403; in this way, in use,each strip 350 may be actuated independently from the other strips 350.

In use, the MEMS actuator 300 of FIG. 9 operates in a way similar towhat described with reference to FIGS. 7-8 , except for the first andthe second spring elements 306, 307 cause rotations of the supportingportion 302 in the opposite direction approximately about the firstsymmetry axis A, and the third and the fourth spring elements 310, 311respectively cause rotations of the first frame 304 and of thesupporting portion 302 in the opposite direction approximately about thesecond symmetry axis B. The voltage sources 400 and 401 are thereforealternately activated, as well as the voltage sources 402, 403.

By providing the deformable arms 330, 332, 340, 342 with a thickness of50 μm and controlling the voltage sources 400-404 with voltages that maybe modulated up to 40 V, it is possible to orientate the supportingportion 302, and, therefore, the lens (not illustrated), by an angle,for example, of +1.57° and −1.57° with respect to the rotation axes A,B.

FIG. 10 shows a further embodiment of the present MEMS actuator. Indetail, FIG. 10 shows a MEMS actuator 500 having a general structuresimilar to the MEMS actuator 300 illustrated in FIG. 9 ; therefore partssimilar to the ones illustrated and described with reference to FIG. 9are designated in FIG. 10 by reference numbers increased by 200 and willnot be described any further.

In particular, the MEMS actuator 500 comprises, in addition to thegeometry described above with reference to FIG. 9 , a first, a second, athird and a fourth torsional arm 620, 621, 630, 631, adapted to connecteach spring element 506, 507, 510, 511 to the first and the secondframes 504, 508, respectively. In particular, the torsional arms 620,621 extend along the first symmetry axis A and are subject to torsionaldeformation about the axis A; likewise, the torsional arms 630, 631extend along the second symmetry axis B and are subject to torsionaldeformation about axis B.

In detail, the first and the second torsional arms 620, 621 extendbetween a deformable arm 530A, 532A of the first and the second springelements 506, 507, respectively, arranged in farther from the center O,and the corner facing the first frame 504 (the corner between theshorter sides 504B of the first frame 504, crossed by the first symmetryaxis A).

Likewise, the third and the fourth torsional arms 630, 631 extendbetween a deformable arm 540A, 542A of the third and the fourth springelements 510, 511, respectively, arranged in a farther from the centerO, and the corner facing the second frame 508 (the corner between thesides of the second frame 508, crossed by the second symmetry axis B).

In use, the MEMS actuator 500 of FIG. 10 operates in a way similar towhat described for the MEMS actuator 300 of FIG. 9 .

From simulations, it has been verified that, with respect to the MEMSactuator 300 illustrated in FIGS. 9-13 , the MEMS actuator 500 has ahigher resistance to external loads, as well as a higher resonancefrequency due to the presence of the torsional arms 620, 621, 630, 631;furthermore, it has been verified that the stress generated by anexternal load (for example, a pressure) prevalently concentrates in thetorsional arms 620, 621, 630, 631.

FIGS. 11-15 show subsequent steps of a manufacturing process of the MEMSactuator 100, 300, 500, in particular the deformable arms 130, 132, 140,142, 330, 332, 340, 342, 530, 532, 540, 542 and part of the first andthe second frames 104, 108, 304, 308, 504, 508 facing them. Forsimplicity, hereinafter reference is made to the MEMS actuator 100, inparticular to one of the deformable arms 130 and to a portion of thefirst frame 104 (in particular, one of the longer sides 104A).

In detail, FIG. 11 shows a first wafer 700, having a top surface 700Aand a bottom surface 700B; in particular, the first wafer 700 isprocessed according to manufacturing steps that are similar to whatdescribed in United States Patent Application Publication No.2014/0313264 A1, incorporated by reference. Consequently, the steps formanufacturing the first wafer 700 common to the above mentioned patentare briefly outlined hereinafter.

The first wafer 700 comprises a substrate 702, of semiconductor material(for example, silicon); the first insulating layer 704, extending on thesubstrate 702; the membrane layer 706, extending on the intermediatelayer 704; the second insulating layer 180 of FIG. 6 ; and a first stackof layers 710, extending over the top surface 700A.

In detail, the first and the second insulating layers 704, 180 areformed according to known growth or deposition techniques, for examplethermal growth, and have a thickness comprised, for example, between 0.1and 2 μm. Moreover, the membrane layer 706 is epitaxially grown and hasa thickness comprised, for example, between 25 and 100 μm, e.g. 60 μm.

The stack of layers 710 comprises layers that are designed to form thefirst electrode 171, the piezoelectric region 172, and the secondelectrode 173 of FIG. 6 and, therefore, are designated in FIG. 11 by thesame reference numbers.

Next, FIG. 12 , the stack of layers 710 is defined according to etchingtechniques so as to form the first and the second electrodes 171, 173,as well as the piezoelectric region 172. Moreover, the second insulatinglayer 180 is defined according to etching techniques to form an opening720, which exposes a portion 722 of the membrane layer 706.

Next, FIG. 13 , a second stack of layers 725 is deposited and definedaccording to deposition and definition techniques.

In particular, the second stack of layers 725 comprises the firstpassivation layer 730; and the second passivation layer 732, extendingon the first passivation layer 730. The first and the second passivationlayers 730, 732 are deposited and defined to form the first and thesecond contact opening 740, 741 and expose, respectively, portions ofthe first and the second electrodes 171, 173.

The second stack of layers 725 further comprises the first and thesecond metallization layers 734A, 734B, deposited and defined accordingto deposition and definition techniques, to form electrical connectionlines.

The second stack of layers 725 further comprises the third passivationlayer 736, which is defined to form the third contact opening 750 and,therefore, to expose at least in part the first metallization layer734A.

Next, FIG. 14 , the contact layer 752 is deposited and defined.

Moreover, in a way not shown, the membrane layer 706 is etched usingknown etching techniques. In this step, the geometry of the thinnerportions of the body 101 (in particular, membranes forming thedeformable arms 130, 132, 140, 142) is defined. Then, trenches (trench755 being visible in FIG. 6 ) are formed in the membrane layer 706.

In detail, an adhesive layer 765 (for example, a coupling adhesive suchas BrewerBOND® 305,https://www.brewerscience.com/products/brewerbond-materials/, having athickness so as to planarize the structure) is deposited on the thirdpassivation layer 736 and on the contact layer 752 using depositiontechniques.

Next, once again FIG. 14 , a carrier wafer 770 is coupled to theadhesive layer 765; for example, the carrier wafer 770 may be a DSP(Double Side Polished) wafer having a thickness, for example, of 400 μm.In this way, a second wafer 800 is obtained, delimited at the top by atop surface 800A and at the bottom by the bottom surface 700B.

Then, FIG. 15 , the second wafer 800 is flipped over and etched from thebottom surface 700B using masking and etching techniques. In particular,the substrate 702 is etched and selectively removed throughout itsthickness (for example, using DRIE) so as to form the substrate portions702A-702C, the cavity 810 and a first part of the trench 755; then, thefirst insulating layer 704 is etched and selectively removed. In thisstep, definition of the geometry of the body 101, in particular of theinternal portion 102, the connection arms 131, 133, 141, 143, and theframes 104, 108, is completed. The cavity 810 is thus formed and exposesat least in part the membrane layer 706 and the adhesive layer 765.

The adhesive layer 765 is then removed via thermal release techniques(e.g. WaferBOND®,https://www.brewerscience.com/products/waferbond-ht-10-10/) so as todetach the carrier wafer 770 from the first wafer 700. Before or afterdetachment of the carrier wafer 700, the first wafer 700 is diced, toform a plurality of adjacent bodies 101.

Next, in a way not shown, the wafer 700 is diced to form the MEMSactuator 100 of FIGS. 4-6 .

The present MEMS actuator and the manufacturing process thereof havemany advantages.

In particular, the body 101 is monolithic and formed in the samestructural, semiconductor material region carrying the piezoelectricactuation elements enabling biaxial rotation of the supporting portion102 (strips 150) and of the optical structures (lens 125). Consequently,the body 101 may be obtained using semiconductor manufacturingtechniques, in a simple, inexpensive and reliable way.

The spring elements 106, 107, 110, 111, 306, 307, 310, 311, 506, 507,510, 511 further enable rotation of the supporting portion 102, 302, 502(and, therefore, of the lens 125) in a fast and precise way. In fact,actuation of the strips 350 is obtained with low actuation voltages (forexample, 40 V); consequently, the power consumption of the MEMS actuator100, 300, 500 is reduced.

Finally, it is clear that modifications and variations may be made tothe MEMS actuator and to the manufacturing process thereof described andillustrated herein, without departing from the scope of the presentinvention, as defined in the attached claims.

For instance, the torsional arms 620, 621, 630, 631 of FIG. 14 may bealso implemented in the embodiment of FIG. 4 .

The invention claimed is:
 1. A MEMS actuator, comprising: a monolithicbody of semiconductor material including: a supporting portion ofsemiconductor material, orientable with respect to a first rotation axisand a second rotation axis, the first rotation axis being transverse tothe second rotation axis; a first frame of semiconductor material,coupled to the supporting portion through first deformable elementsconfigured to control a rotation of the supporting portion about thefirst rotation axis, wherein the first frame has an elongated hexagonalshape, with two first sides parallel to a first symmetry axis and fourend sides extending transverse to the first symmetry axis and a secondsymmetry axis, wherein the first symmetry axis and the second symmetryaxis are parallel to the first and second rotation axis, wherein thefirst deformable elements extend perpendicularly to the first symmetryaxis; and a second frame of semiconductor material, coupled to the firstframe through second deformable elements that are coupled between thefirst and second frames and configured to control a rotation of thesupporting portion about the second rotation axis, wherein the secondframe has a regular quadrangular shape with sides parallel to the endsides of the first frame, wherein the second deformable elements extendparallel to the first symmetry axis; and wherein the first and seconddeformable elements carry respective first and second piezoelectricactuation elements.
 2. The MEMS actuator according to claim 1, whereinsides of the second frame extend at 45° with respect to the first andsecond symmetry axes, and the first and second deformable elementsextending at 45° with respect to the sides of the second frame.
 3. TheMEMS actuator according to claim 1, wherein the first and seconddeformable elements are each formed by first and second elasticelements, the first and second elastic elements of the first deformableelements being arranged on opposite sides of the supporting portion, andthe first and second elastic elements of the second deformable elementsbeing arranged on opposite sides of the first frame, the first andsecond elastic elements having a serpentine shape, wherein the first andsecond elastic elements of the first deformable elements extendtransverse to the first symmetry axis and the first and second elasticelements of the second deformable elements extend transverse to thesecond symmetry axis.
 4. The MEMS actuator according to claim 1, whereinthe first and second deformable elements are each formed by first andsecond elastic elements, the first and second elastic elements of thefirst deformable elements being arranged on opposite sides of thesupporting portion, and the first and second elastic elements of thesecond deformable elements being arranged on opposite sides of the firstframe, the first and second elastic elements having a serpentine shape,wherein the first and second elastic elements of the first deformableelements extend transverse to the first symmetry axis and the first andsecond elastic elements of the second deformable elements extendtransverse to the second symmetry axis.
 5. The MEMS actuator accordingto claim 2, wherein the first and second deformable elements are eachformed by first and second elastic elements, the first and secondelastic elements of the first deformable elements being arranged onopposite sides of the supporting portion, and the first and secondelastic elements of the second deformable elements being arranged onopposite sides of the first frame, the first and second elastic elementshaving a serpentine shape, wherein the first and second elastic elementsof the first deformable elements extend transverse to the first symmetryaxis and the first and second elastic elements of the second deformableelements extend transverse to the second symmetry axis.
 6. The MEMSactuator according to claim 3, wherein the first and second elasticelements of the first deformable elements comprise: respective first andsecond deformable arms, carrying the first piezoelectric actuationelements; and respective first and second connection arms, connectingopposite ends of respective successive first and second deformable arms,thereby forming the serpentine shape.
 7. The MEMS actuator according toclaim 3, wherein the first and second elastic elements of the seconddeformable elements comprise: respective third and fourth deformablearms, carrying the second piezoelectric actuation elements; andrespective third and fourth connection arms, connecting opposite ends ofrespective successive third and fourth deformable arms, thereby formingthe serpentine shape.
 8. The MEMS actuator according to claim 3,wherein: the first and second elastic elements of the first deformableelements are coupled to the supporting portion by respective first endsand to the first frame by respective second ends; the first and secondelastic elements of the second deformable elements are coupled to thefirst frame by respective first ends and to the second frame byrespective second ends; the first and second ends of the first andsecond elastic elements of the first deformable elements aresymmetrically arranged with respect to the second rotation axis; and thefirst and second ends of the first and second elastic elements of thesecond deformable elements are symmetrical with respect to the firstrotation axis.
 9. The MEMS actuator according to claim 3, wherein: thefirst and second elastic elements of the first deformable elements arecoupled to the supporting portion through respective first ends and tothe first frame through respective second ends, the first and secondelastic elements of the second deformable elements are coupled to thefirst frame through respective first ends and to the second framethrough respective second ends, the first and second symmetry axesdefine a center of the MEMS actuator, and the first and second ends ofthe first and second elastic elements of the first and second deformableelements are arranged rotated by 180° with respect to the center. 10.The MEMS actuator according to claim 8, further comprising first andsecond torsional arms of semiconductor material, wherein the firsttorsional arms extend between the first elastic elements and the firstframe, and wherein the second torsional arms extend between the secondelastic elements and the second frame.
 11. The MEMS actuator accordingto claim 10, wherein the first torsional arms extend along the firstsymmetry axis and the second torsional arms extend along the secondsymmetry axis.
 12. The MEMS actuator according to claim 1, wherein thesupporting portion has a first and second larger surfaces and comprisesa central opening, the supporting portion supporting a lens.